Understanding the difference between Linear, Buck, Boost and Direct Drive drivers

a mass market driver with boost should be buck+boost, and i can understand the complexities of such drivers. good explanation also! but the driver in dicussion with lightmalls would have been boost only (DD at Vin>Vf). in other words, a specialised driver for lights which can withstand DDing an emitter (i.e. 5A+).

at high current would a boost driver never reach the required efficiency to work from a single li-ion? or something else, like lack of commercial ics? i can’t even find any mass produced step-up voltage converters for this range and output.

I agree, I changed the OP accordingly. Thanks! :slight_smile:

Buck-Boost drivers are also less efficient than just a Buck.
If he wants to drive a XM-L2 at 4A or more he should consider having two Li-Ion cells in series to get 7.4V and using a buck converter… However it will be bulky to be able to withstand 4A.

Again, as long as the input voltage is below 6V even with fully charged cells, the driver won’t fail. But it will be wasteful and run hot.
Let’s do some maths:
Your 4*AA are at about 5V during discharge.
The LED needs about 3.3V
that means that the driver needs to lower the voltage of 1.7V!
3.3/5=66% efficiency! That’s not really good… And that means that 37% of the power is lost in the driver and heats it up.
Use a dummy cell and efficiency will go up, reducing the heat in the driver and you’ll still have the same brightness.

The general problem with a boost drivers is that it always need more input current than it delivers as output current.

Efficiency varies with design, but because the current is high it will have high ohmic losses.

i see in tests of some high discharge cells that outputs of 10A @ >3V are sustainable, so i assumed in theory a boost driver could continue to regulate on a single cell. if the loss is primarily ohmic then i presume it is linear. the size constraints make it difficult i guess.

Just a FYI/opinion:

Advantages of the direct drive drivers are also that there is no tint shift between turbo and low. Or at least very very little. True constant current drivers show a huge (to me, it might be individual) tint shift so the color of things look "off" in lower modes. I especially notice this with high CRI emitters and very cool white emitters.

And the draining of the battery does change the tint a little but it is almost unnoticeable over time. What is not unnoticeable is the rapid decline in light. At least rapid enough for me to notice it and get another battery ready to load.

And I disagree with the first line of the conclusions. The one about 1 li-ion and 1 emitter.

For the above reasons.

A linear driver can use PWM to make low modes. In that case there is no tint shift.
It’s the linear drivers that are truly constant current, even in low modes that cause a tint shift. That’s the disadvantage of constant current driver, the advantages are a better efficiency and no flickering.
A driver like this one is linear, which is good because the brightness will not vary when the battery discharges. But it also uses PWM to make lower modes. So no tint shift.
In my opinion this is better than a direct drive drivers because the brigtness stays constant. I don’t see any advantage to the cheap direct drive drivers that have a 0.2ohms resistor to limit current to safe levels…
I have to admit that I may be biased because I personally don’t care about tint shift, but I really care about the improved efficiency of true constant current drivers. I may add a sentence about tint shift and PWM to clarify things out.

@Islisis, i think you are right, the size is the problem, nothing else.

I don’t think I ever posted in this thread to thank you lagman, but I do point people over here every little while. One of these days I’ll make some suggestions of my own for sprucing things up, but I think you’ve done a good job!

As my post starts by pointing out. It is an opinion.

I always accepted the inevitable: My batteries run out.

How does the new Cree MT-G2 and XHP 6v+ emitters affect things

I notice that the article calls out single cell and single emitters a lot but I think it might be more accurate to limit the ideas to Voltage over and under the emitter requirements. Point being that with the MT-G2 emitters there are lots of standard designs that are exceptions to the single cell nomenclature.

$.02 worth

A long time ago I said that I’d like to contribute. Sorry for the wait folks. Today I took an hour or two and did some editing.

While I think more than half of lagman’s text has been rewritten, the significance of the edits varies. Some editing was rather heavy, some just minor reformatting. One or two things were incorrect in minor ways or simply misleading. It’s still not perfect, and it might have gotten slightly more technical but I think it’s an improvement. The overall structure is still lagman’s!

Much of my editing was to make the guide more compatible with discussing drivers for 6v and 12v LEDs. I removed a lot of mentions of LED voltage being ~3v. I also added context in other places where 3v or a single li-ion is used as an example. I also added detail and context where I thought it most appropriate.

I did my best to keep it short and sweet. I left out plenty of things. Unfortunately I was unable to avoid increasing the word count by about 75%.

I welcome any feedback on my edits or the overall state of the guide…

Thank you wight! I've read through the wiki but this is better.

Thank lagman. The initial writeup took the most time and effort.

I see that my thread is not dead yet. :slight_smile:
Thanks to you all for the support!
You did a good job wight. It is true that I didn’t take into consideration multiple LED dies like the new Cree XPH. That being said the theory remains the same.

I have one minor point on which I disagree though:

I think it is wiser to consider the “recommended operating conditions” which is 6V instead of the “absolute maximum rating” which is 7V. But in any case two lithium batteries in series will have a max voltage of 7.4V which is sure to damage the chip.

Thanks lagman.

Let me make sure you have a good handle on actual 7135 operation / implementation: There are two voltage specifications for the 7135. “Input Voltage (Vin)” and “Output Voltage (Vout)”.

  • Input voltage is measured in the normal way, Vdd - GND = Vin. [Since GND is zero we may simply say that Vdd = Vin.] When we are powering this from an MCU you may expect for “Vin” for the 7135 to be in the 2.0v to 5.5v range. We typically regulate the voltage to our MCU at less than 5.5v using an LDO or Zener and the Vdd pin is powered by the MCU. For this reason we may generally ignore the Vin specification as we are always in the “normal” range.
  • Output voltage is measured in this way: Out - GND = Vout. [Again, since GND is zero we may simply say that the voltage at the Out pin = Vout.] Vout is the voltage left over after the LED (Vf), so in order to get Vout we do Vbat - Vf = Vout. In an 2s Li-Ion (2x 4.2v) powered 8.4v MT-G2 light the input voltage will be no higher than 8.4v and at 3A output we may expect an LED Vf around 6.4v. So Vbat - Vf = 2.0v. Again, no problem.

So operating the 7135 in high voltage lights is not a problem. The key (as I’ve tried to express) is the amount of “extra” voltage beyond LED Vf you have. Running an XM-L on 2s Li-Ion is actually within spec for the 7135 in terms of voltage! The problem remains that the wattage would be far too high in that scenario.

Ok, I agree with you. I wasn’t thinking that the VDD of the 7135 chip is powered by the MCU… :slight_smile:

:slight_smile: